Natural gas, which is essentially methane, generally contains significant amounts of nitrogen contaminant as it emerges from a reservoir. The nitrogen may be naturally occurring and/or may have been injected into the reservoir as part of an enhanced gas recovery or enhanced oil recovery operation. Other contaminants which may be present in the natural gas from a reservoir include water, carbon dioxide, helium, hydrogen sulfide and higher hydrocarbons. In order to produce natural gas of a purity suitable for commercial use, the reservoir gas stream must be separated into components. Often the separation is by cryogenic rectification using either a single column or a double column separation plant. Generally, the nitrogen fraction comprises from 10 to 70 percent of the feed to the separation plant.
Generally the purified methane gas product from the cryogenic separation is introduced into a pipeline for delivery to end users and, in order to do so, the methane product gas must be compressed to the pipeline pressure which is generally at least about 500 psia. This methane product gas compression is quite costly and it is therefore desirable to eliminate or at least reduce methane product gas compression requirements.
Accordingly it is an object of this invention to provide a method for the separation by cryogenic rectification of nitrogen and methane wherein at least some methane gas product is produced at higher pressure thereby reducing the amount of methane gas product compression which is necessary to allow introduction of the methane gas product to a pipeline.